Søyland, C., Hassfjell, S.P. and Steen, H.B. A New Alpha-Particle Irradiator with Absolute Dosimetric Determination. A new experimental setup for uniform α-particle irradiation of cells in vitro is described. The α-particle irradiator is based on a radioactive 212 Pb/ 212 Bi source. In the experimental setup proposed, cells are grown directly on a polylysine-coated track-etch material that forms the base of custom-made cell dishes. Alpha-particle irradiation is done through the base of the dish. Immediately prior to irradiation, the cell dish is scanned under a microscope, and images of cells with the corresponding coordinates are saved. After irradiation and after the biological end point under study has been determined, the cell dish is etched to develop α-particle tracks in the dish base. A microscope image series of α-particle track images is obtained by accurately revisiting every original (preirradiation) cell position in the track-etched dish. The number of α-particle traversals of each individual cell is scored by mapping images of α-particle tracks onto the images of cells recorded prior to irradiation. The uncertainty of the α-particle hit determination is 0.9 μm. The procedure described thus presents a method for radiobiological experiments with absolute, rather than statistical, cell dosimetry.